This invention relates to an endoscope that is equipped with a video readout for providing television pictures of a remote target and, in particular, to an automatic signal level control for use in a video-equipped endoscope.
With the development of solid state image sensors, sometimes referred to as charge coupled devices (CCD), it is now possible to equip the insertion head of an endoscope with an extremely small video camera that permits the head to be passed into confined regions that have heretofore been inaccessible to this type of viewing equipment. The camera generally contains a single black and white image sensor that is capable of integrating the recorded image data that is acquired during each imaging sequence. Full color video pictures of the target can also be created using a single imager by illuminating the target sequentially with light of primary colors to produce a series of color separated images. The primary colors of red, green and blue are generally used for this purpose. After each color separated image is recorded, the image data is clocked out of the image sensor and delivered to a video processor where the combined data is placed in a format suitable for display on a television screen.
As explained in greater detail in reissued U.S. Pat. Nos. Re. 31,289 and Re. 31,290 to Moore et al, three individual lamps can be used to illuminate the target with light of the primary colors to provide the desired color separation. In a further U.S. Pat. No. 4,074,306 to Kakinuma et al, there is described a video-equipped endoscope that employs a single lamp that operates in conjunction with color wheel to provide color separation. In the latter embodiment, the color-separated images are laid down in series, one over the other, upon a Braun tube to create a color picture of the target so that registration of the images and attaining proper balancing of the primary colors is extremely difficult. A single-lamp illumination system that avoids the difficulties found in the Kakinuma et al device is disclosed in copending U.S. application Ser. No. 487,070 filed April 21, 1983 in the name of Sarofeen et al. In the Sarofeen system, the output of the lamp is closely controlled during each color separation imaging sequence so that the color appearing in the video picture are proportionally balanced to provide a faithful rendition of the original scene.
As is well known in the art, the viewing head of the endoscope is mounted at the distal end of the insertion tube and is generally required to operate within a very confined area having little, if any, ambient light. This is particularly true when the instrument is used in a medical application. Light is generally brought into the image region by means of a fiber optic bundle. As the head is moved towards or away from a given target situated in this relatively confined region, the amount of light reflected from the target back to the image sensor will vary dramatically. Although an automatic gain control circuit may be included in the video section of the instrument, this in itself may not be sufficient to prevent the sensor from becoming saturated with light (blooming) when the head is moved close to the target. By the same token, the amount of light that can be transmitted into the target region by the fiber bundle is limited and consequently, the gain of the video signal can fall to a level where background noise is visible when the head is moved a relatively short distance away from the target.